Method of corneal analysis using a checkered placido apparatus

ABSTRACT

A method for analysis of the curvature of the surface of a cornea using a checkered placido comprises, projecting the image of the checkered placido onto a patient&#39;s cornea, detecting the image of the checkered placido reflected off of the cornea, detecting a plurality of nodal points from the reflected image, determining the mean curvature at a plurality of nodal points and analyzing the mean curvature at a plurality of nodal points in order to produce a graphic display of the estimated actual curvature of the cornea.

This is a continuation in part of application Ser. No. 07/891,961, filedJun. 2, 1992 now abandoned.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is related to the patent application for a"VIDEO TO PRINTER INTERFACE METHOD AND APPARATUS" by Edwin J. Sarver,Henry M. D'Souza, Steven Woo filed concurrently on Apr. 10, 1992. Thispatent application is related to the design patent application for"DESIGN FOR AN ABSOLUTE DIOPTRIC SCALE REPRESENTATION" filedconcurrently with this application on Apr. 10, 1992 by Edwin Jay Sarver,Ph.D. and assigned to EyeSys Laboratories, Inc. This patent applicationis also related to U.S. Ser. No. 07/817,868, "A" Contact LensSculpturing Device" by Wakil, D'Souza, Baumgartner, and Carbonari andassigned to EyeSys Laboratories, Inc. (pending divisional application of607,640 filed Jan. 7, 1992); U.S. Ser. No. 07/818,659, "A Method ofUsing A Placido" by Wakil, D'Souza, Baumgartner, and Carbonari andassigned to EyeSys Laboratories, Inc. (pending divisional application of607,640 filed Jan. 7, 1992); and U.S. Ser. No. 07/819,364, and "A"Placido Apparatus" by Wakil, D'Souza, Baumgartner, and Carbonari andassigned to EyeSys Laboratories, Inc. (pending divisional application of607,640 filed Jan. 7, 1992).

MICROFICHE APPENDICES FOR COMPUTER PROGRAM SOURCE CODE LISTINGS

This patent specification includes a microfiche appendix. The appendixis a source code listing for the CORNEAL ANALYSIS SYSTEM software, whichconsists of 11 microfiche, for a total of 997 frames.

BACKGROUND OF THE INVENTION

A number of forms of eye surgery including lamellar corneal surgery,keratomileusis, epikeratophakia, cataract surgery, penetratingkeratoplasty, corneal transplantation radial keratotomy as well as laserrefractive keratectomy involve a consideration of corneal surfacetopography. In radial keratotomy, for example, a number of cuts are madeinto the cornea in order to change its curvature and correct refractivepower so that images focus closer to the retina, if not upon it for bestvisual acuity. It has been reported that after radial keratotomy "about55 percent of the patients function without glasses and the remaining 45percent have some degree of improvement." Origination of the techniqueof radial keratotomy and other techniques in refractive surgery aregenerally credited to Dr. Svyatasklav Fyodorov of the Soviet Union whois reputed to have performed thousands of such operations.

While ophthalmic surgery is often successfully performed, the resultsobtained have been subject to variation occasioned by the particularoperating "style" of the individual surgeon which dictates the number,location and depth of incision. Elements of subjective judgment areparamount. It would be useful to provide a device that could assist thesurgeon in more quantitatively assessing pre-operative andpost-operative corneal contours.

The present system relates to improvements in the art ofphotokeratometry and more particularly to the use of digital imageprocessing techniques to ascertain the radius of curvature, refractivepower and contour of the cornea. A keratometer is an instrument fordetermining the curvature shape of the corneal surface which generallyuses a Placido or other illuminated target that is centered around thepatient's line of sight. The reflection of a Placido or otherilluminated target by the patient's cornea or by the tear film on theanterior surface of the cornea is subsequently analyzed to determine thesurface contour of the eye.

The technique in modem form dates from the early thirties when the Zeissoptical company of Germany introduced a "Photo Keratoscope". In general,the art has required the image reflected by the eye to be photographedand the image on the film measured in a second step to derive thequantitative data from which the contour map is generated.

Recent improvements have been in the area of automating thisphotogrammetric analysis by re-imaging the photograph with a televisionapparatus and digital signal conversion. After digitization, computeranalysis of the resultant information is performed with conventionalimage analysis algorithms. This type of data analysis is computerintensive and the image formed by the television system contains a largeamount of redundant and extraneous information. For adequate resolutionthe sampling rate must exceed the data frequency by at least three toone, thus generating a huge number of data points for mathematicalanalysis. Consequently the systems are costly, complex, slow and oftenlack real resolution in the image analysis. Other means have been usedfor clinical measurements such as direct casting of the eye surface inplastic or wax and coating the cornea with talcum powder and projectinga grid on this surface for photogrammetric analysis.

The initial development in keratometry came from Gullstrand in 1896.Gullstrand disclosed the foundation for the current technology but hisapparatus had no provision to compensate for aberrations in the opticalsystem other than limiting the photographic coverage of the cornea to a4 mm area. As a result, multiple exposures and calculations werenecessary to map the corneal surface. Much of the modem technique wasdeveloped by Amsler in 1930 and embodied in his "Photo-Keratoscope"which also required measurement and calculation as a separate step toderive the corneal shape data.

At present, the clinical standard is the Bausch and Lomb Keratometer,which is sold commercially. The Bausch and Lomb Keratometer onlymeasures the average of the corneal radius in two meridians of thecentral 3 mm "cap" of the cornea. The standard technology does notprovide total surface topography of the cornea and thus is inadequatefor many diagnostically significant abnormalities, contact lens fitting,or the needs of ophthalmic surgical procedures. In addition, the priorart technique is cumbersome and involves great potential for error.

The standard instrument which is in most common use for central opticalzone shape measurement is the Bausch and Lomb Keratometer. Severalcompanies offer similar devices with similar principles of operation. Inthese devices a single Mire image is projected on a small centralportion of the anterior surface of the cornea usually 3 mm in diameter.The user is required to operate several controls to bring the opticallysplit Mire images reflected from the cornea simultaneously into focusand alignment. In addition, the operator manually records the dataobtained at two perpendicular axes. Other instruments are alsoavailable, such as the Haag-Streit Javal Schiotz device which measuresonly one axis at a time, but is slightly easier to use and tends to bemore accurate in practice than the Bausch and Lomb system. In additionthere exists a photographic system made by International DiagnosticInstrument Limited under the trademark "CORNEASCOPE" (and a similarsystem made by Nidek in Japan), as well as autokeratometers by severalmanufacturers. The CORNEASCOPE produces instant photographs of thereflection of a Placido disc and requires a second instrument separatefrom the camera assembly to analyze the data. This system is fairlyaccurate, but expensive and tedious to use. The autokeratometers all arelimited to a single zone of approximately 3 mm diameter and, in caseswhere the magnitude of the astigmatism is low, are inaccurate in theirassessment of axes of astigmatism. Also available are threecomputer-direct systems which use conventional image analysis algorithmsin conjunction with a mini-computer. These are the Corneal ModelingSystem (CMS) introduced in 1987 by Computed Anatomy, Inc. of New York,N. Y. and the ECT-100, introduced into the market by Visioptic ofHouston, Tex. and a system using light emitting diodes disposed inconcentric rings built by Zeiss of Germany. The Placido disc-phototechnique is superior to the Bausch and Lomb Keratometer because of themuch greater amount of corneal surface analyzed from the Placidoreflection as opposed to the mires of the Keratometer.

A number of patents have been issued that relate to keratometers. U.S.Pat. No. 3,797,921 proposes the use of a camera to record the Placidoreflection from a patients eye. From this photograph, the radius ofsurface curvature of the cornea is determined at several points andcalculated using a complex computer system. The use of a ground glassfocusing screen with the small aperture of the optical system and largelinear magnification makes use difficult and requires a darkened roomfor operation.

U.S. Pat. No. 4,440,477 proposes a method and device for measuring thecorneal surface, comprising a slit lamp for illuminating the cornealsurface, a camera for recording the reflection from the corneal surface,and a processor to calculate the image distance and the radius ofcurvature of the eye. The operation of the processor is not detailed inU.S. Pat. No. 4,440,477.

A more recent entry into the market is the "Corneal Modeling System"manufactured by Computed Anatomy Incorporated of New York which uses alight cone Placido target in conjunction with a "frame grabber" todigitize and store for conventional image analysis the pictorial data.The Placido is in cylindrical form and illuminated from one end. Thiscylindrical Placido maintains a small aperture optical system creating alarge depth of field of focus for the imaging system and, consequently,requires a sophisticated focus determining apparatus to assure accurateand reproducible image evaluation. This system is said to producecorneal thickness data using a scanning laser, as well as the surfacecontour but is very expensive and does not lend itself to clinicalapplications which are increasingly cost driven.

The prior art systems discussed above tend to be both expensive anddifficult to use. Many of the prior art devices have a significantpotential for error, due to complexity of the calculation, the imagingof the corneal surface and the difficulty in operating these systems.

Since even a normal human cornea will not be perfectly spherical, theilluminated rings will generally be reflected from the corneal surfaceas a pattern of shapes variously distorted from the circular. The datapertaining to the coordinates of points in the two-dimensional videoimage is processed to define a three-dimensional corneal surfaceyielding the equivalent spherical radius of curvature (or dioptricpower) for each of the acquired points.

SUMMARY OF THE INVENTION

Accordingly, there is provided herein a new technique for image analysisthat provides full topographical mapping of the cornea, with almostinstant display of the corneal radius of curvature at enough points topermit accurate assessment of the surface shape. The improved photokeratometer includes a transilluminate target or "Placido", which isreflected by the surface of the eye to be examined. A CCD camera andlens system is mounted behind the Placido so that the optical axis iscoincident with the visual axis of the eye being examined and isgenerally centered in the target member to provide an image of thereflection of the target by the eye. The image information of multiple"rings" on the cornea from the CCD camera is then captured by a framegrabber board and processed by an edge detection algorithm to derive thelocus of image brightness discontinuities which are associated with thetarget reflection from the eye. These image points are, in turn,transferred to storage in the internal memory as digital representationsof the x, y locus of the image bright/dark transitions representing thePlacido ring edges.

The stored data associated with the CCD image of the target reflectionare then treated by an image processing algorithm in a conventionalelectronic computer to derive the surface contour of the eye and togenerate the display of the derived shape information for use by theoperator. The Multi-Functional Corneal Analysis System described hereincan serve as a sensitive method to determine proper contact lens fit bymeasuring the shape of both front and back surfaces of the contactlenses and comparing these shape measurements with the shape of the eyeto which the said lens is to be applied.

An illustrative system in accordance with the invention the EyesysMulti-functional Corneal Analysis System which combines the features ofan automatic keratometer, photokeratoscope and corneal topography deviceinto a single instrument. Comprehensive keratometric results andquantitative corneal surface measurements provide multi-functionalcorneal evaluation capabilities. Multiple analysis routines offerinformation from basic keratometric readings to intermediate zone valuesand graphics to full surface topography color mapping. An easy to usejoy-stick and positioning aid provides precise patient alignment andimage focus. User-friendly menus guide users to quick and reproducibleexams. An on-line operators manual provides rapid assistance. For mostexams, processing time is under 10 seconds for 360 meridians. Cornealinformation is reported as numerical values with graphic presentationsfor the 3 mm, 5 mm and 7 mm zones, corneal contour profile graphics ofany two meridians and topographical color surface maps according toeither dioptric power or millimeter radius of curvature. Up to foursurface maps can be displayed for comparative analysis. Patient examscan be archived to hard disc or floppy disc and recalled at any time.Permanent records may be produced via optional Polaroid camera or colorgraphics printer.

An illustrative system in accordance with the invention utilizes aunique data-acquisition design to perform rapid, cost-effectivequantitative photokeratoscopy. The system obtains a complete 360 degreemeasurement (approximate corneal zone diameter 0.9-9.5 mm) with only asingle data-acquisition "shot," eliminating the need for camerarotation. The system has a more precise and user recognizable focusingtarget and improved optics over other systems known in the art, whichfurther enhances the accuracy and reproducibility of corneal topographicprofiles. The interval between electronic data capture and completedisplay for all meridians is less than 10 seconds. The system ispackaged either as a single tabletop unit with a base dimension ofroughly 18"×23" which includes an integrated IBM-compatible computer anda photographic port so that standard photokeratoscopic photographs onPolaroid or 35-mm film can easily be obtained or as a modular unit on amobile pedestal with dimensions of 32"×24" with computer housingseparate from photokeratoscope. Video output is available for videoimage storage if desired. In addition to standard numerical displays,new color graphics for corneal profiles and for isodioptric color-codedcontour maps can be selected. The system in accordance with theinvention is easy to use and therefore suitable for use in a standardclinical setting. Its data-acquisition design provides rapid datacapture and display and offers distinct advantages for clinical andresearch applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of the system.

FIG. 2 is a cross section of the system.

FIG. 3A-3M are a detail of the system menus and graphic presentations.

FIG. 4 is a diagram of optical principles.

FIG. 4A and 4B are a graphic presentation of the operation of thefocusing aid.

FIG. 5A-5C are a detail of the construction of the focusing aid.

FIG. 6A-6D are an operational sketch of the focusing aid.

FIG. 7 is operational depiction of the optical assembly and patientpositioning assembly.

FIG. 8A-8I are a mechanical drawing of the optical assembly housing.

FIG. 9 is a schematic of the power supply.

FIG. 10A-10K a block diagram and schematic and PAL equations for theframe grabber board.

FIG. 11A and 11B are the optical path layout and design methodology.

FIG. 12A-12K the system menus and high level description of thesoftware.

FIG. 13 is a cross section of the eye.

FIG. 14 is a front view of the eye.

FIG. 15 is a graphical representation of the checkered Placido apparatusin operation.

FIG. 16 is a front view of a checkered Placido apparatus.

FIG. 17 is a rearview of a checkered Placido apparatus.

FIG. 18 is a diagonal view of a checkered Placido apparatus.

FIG. 19 is a front view of a checkered Placido apparatus.

FIG. 20 is a front view of a Placido.

FIG. 21 is a side view of a checkered Placido image projected onto acornea and an expanded view of a selected nodal point.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

I. System Overview

FIGS. 13 and 14 illustrate the more important features of the eye asthey relate to keratometry. The pupil of the eye is defined by thecentral area surrounded by the iris. The iris opening size is controlledby the autonomic nerve system in relation to the brightness ofillumination as well as other factors and may be as small as onemillimeter in diameter in bright light to five millimeters in diameterin dim light. The constriction of the iris in bright light also providesan increase in depth of focus such as is observed in conventionalphotography. The reflection of the one or more concentric rings of thePlacido off of the anterior corneal surface will appear as more or lesscircular bright rings superimposed on the pupil and iris when viewed bythe television camera. The interior of the eye is shown as a horizontalcross-section to show the more important structures. The globe isenclosed in a semi-rigid white membrane called the sclera. Thetransparent membrane at the front is called the cornea. The cornea is athin membrane which is supported in shape by the pressure of the fluidbehind the membrane and in front of the crystalline lens. The lens issupported by a system of filaments and muscle tissue which cooperate tochange its thickness and, in consequence, the focal length of the lens.

The primary focusing power of the optical system of the eye is providedby the light refraction curvature of the cornea and the fluid fillingthe anterior chamber, while the lens serves to permit the change ofplane of focus from near objects to distant scenes. The light enteringthe eye through the iris opening is brought to focus on the surface ofthe retina which lines a large portion of the inner globe and containsthe photo-receptor cells. These cells are of two general types, rods andcones. The rods predominate in the areas peripheral to central image andare highly sensitive to light but devoid of color sensitivity. The rodsprovide "scotopic" or night vision. The cones predominate in the centralretina and in the "fovea", where critical central vision takes place.The center of vision is located in the fovea which is displaced from theoptical axis of the eye by some five to seven degrees. Because thesurface of the cornea is not a perfect spherical section the curvatureof the surface is asymmetrical around the center of vision or visualaxis and must be taken into consideration in keratometry.

As noted in U.S. Pat. Nos. 3,542,458 and 4,440,477 the reflection of anobject in a convex mirror will produce an image which is "virtual"(cannot be formed on a screen, but can be viewed directly) erect, andreduced in size by an amount which is a function of the radius ofcurvature of the mirror. In this system, the tear film and/or thesurface of the cornea acts as such a mirror. The formula often used todefine the light reflected from a transparent surface is dependent uponthe index of refraction of the optical medias involved.

The commonly used values of the indices of refraction, n, for the threemedia of transmission in this case, air to tear film and cornea are1.000 for air, 1.333 for the tear film and 1.3375 for the cornea. Thereexists approximately a two percent reflection at both of these opticalinterfaces, i.e. the air/tear film and tear film/anterior cornealsurface. The small thickness of the tear film places both reflections inclose proximity so that they are indistinguishable from each other forinstrumental purposes. As a result these reflections are lumped togetherfor clinical applications. However, the small amount of light in thereflected pattern influences the system design, as discussed below.

The anterior surface of the normal cornea is not quite spherical, as itappears to have been assumed in the construction of many of the priorart devices such as the Bausch and Lomb Keratometer, but is more nearlyan ellipsoid. The central two or three millimeters of the normal corneadoes conform reasonably to the spherical form so the simplistic modelwill serve to illustrate the optics of the system for rays at or nearthe common optical axis.

The user is most often interested in data presentation in terms ofdiopters of focusing power of the cornea. The radius information canthen be converted to this form in the commonly use formula as follows:

    d=(n-1)/r

where the index of refraction of the cornea n is assumed to be 1.3375and the radius of curvature of the corneal surface r is expressed inmeters. It should be noted that there is not an agreement on the actualvalue of the effective index of refraction of the cornea to be employedin keratometry and that the calculation of corneal curvature in dioptricform also involves optical correction factors to compensate for theeffectively negative "lens" formed by the rear surface of the cornea. Inpractice the value of index of refraction used by several systems forthis conversion range from 1.332 (Zeiss) 1.336 (American Optical) to1.3375 (Haag-Streit and Bausch & Lomb). The "normal" range of curvaturein the central zone ranges from 7.2 to 8.3 mm with a mean value of 7.8mm. Some representative values for the Bausch & Lomb instrumentsconverting the readings into diopters are shown in the following table:

    ______________________________________                                        Dioptral curvature                                                                          Surface radius in mm                                            ______________________________________                                        61.0          5.53                                                            60.0          5.63                                                            47.0          7.18                                                            45.0          7.50                                                            44.0          7.67                                                            42.0          8.04                                                            41.0          8.23                                                            ______________________________________                                    

From the foregoing it follows that the conversion of the data intodioptric form is not difficult and involves the use of a selectedconstant but that the data so expressed is subject to variable errorinherent in the technique. The common keratometer has been used for manyyears with data in dioptric form, even though the magnitudes are notprecisely accurate. The choice of display form either in diopters ormillimeter radius of curvature is selectable in this system to permitthe user to choose between the more accurate and the more common form.The display of the derived data may be in graph form for ease ofassimilation and application by the user.

The data of interest to the user is generated from the pixel radii ofeach chord of Placido ring reflection in the acquired image in any ofthe possible directions from center. The millimeter radius of curvatureand dioptric curvature of the surface at each of these points is thenprovided to the user for his evaluation. The keratometer known in theart and in common use measures two perpendicular meridians at eachselected angle and produces data in the form of "K1, K2", cylinder andaxis. These terms refer to the average dioptric curvature from bothsides of visual axis in each of the two meridians which have thegreatest and least curvature, assumed to be 90 degrees apart in"regular" astigmatism the magnitude of the difference between the two,and the angle relative to the horizontal of the larger of the two. Theterms are commonly used and are recognized by the user as definitive ofthese descriptive elements as derived by conventional keratometry. Theaxis can either be measured or assumed to be regular (90 degrees apart),however, in today's applications more comprehensive data is necessary. Kvalues are obtained for a full 360 degrees by a process of repeatedmeasurement and recordation.

To reduce the amount of data required to define the ring image size inradial terms, only those pixel loci which define a change of brightnessgreater than a threshold value are stored. Each ring reflection producesone data point at each reflection edge. These points can be used todetermine the actual locus of the center of the ring reflections. Theoptical system is preferably provided with an optical fiber whichdefines the optical center of the system and provides a bright point oflight for the patient to fixate upon. The reflection of this small pointfrom the cornea provides a true center from which all measurements aremade. Furthermore the numerical scatter of the data points is a functionof the focus and overall image quality which permits the evaluation ofeach measurement for minimum acceptable quality. The decision to rejectany measurement which does not fulfill the quality standard is set intothe software. This is due to the requirement that the object distance beknown and fixed for accurate data analysis. Small errors in focus candegrade the measurement and so an optical system with a small depth offield of focus and a software scatter determination are used to insureaccuracy. The central fixation target reflection from the optical fiberis also examined for relationship to the true center of the picture andif the image is decentered in either axis by a predetermined amount themeasurement is invalidated. The shadow cast by the nose, brow, lashes,etc. as well as the lid margin which may lie within the camera fieldwill cause some data points to be missing from the theoretical maximumnumber. The lash shadows will not completely obscure the area to bemeasured and so some minimum number of valid points may be selectedwhich will permit the areas thus partially masked to be defined with alarge degree of confidence. The entire picture is examined forbrightness transitions in this manner and the axis determined bymathematical algorithms in the computer. Given that, for example, theimage resolution of the system provides a pixel size, Placido imagereferred, of 0.014 mm (750 pixels=10 mm so one pixel=1/75 mm or 0.0133mm) then an estimate of the minimum curvature difference and radialinterval detectable by the system can be derived.

For best accuracy, each instrument should be calibrated periodically tocompensate for minor differences in system magnification and linearityto obtain maximum accuracy of the derived data. For this reasoncalibration means preferably are provided as a part of the computersoftware and the user may check the calibration and reset the tablevalues at any time.

Referring now to FIG. 4, the optical theory diagram shows some of therelationships which are of interest in the present system.

An object (the cornea of the eye to be measured) with a size h is imagedby reflection at plane d with an image size h'. The magnification isderived by the usual formula:

    m=hl'h

The focal length of a convex mirror is negative and equal to one-half ofthe radius of curvature. The sum of the reciprocals of the object andimage distances is equal to the reciprocal of the focal length. Thesetwo can then be combined to the form:

    1/o+1/i=1/-f=-2/r

or

    i=or/ 2(or)-r!

It follows that the remainder of the image is formed in a similarfashion and that the same figure applies in any meridian. (Theseformulae are only true for rays which are very close to the opticalaxis). From the size of the object, size of the image, the distance andthe optical magnification, the radius of curvature can be calculated asfollows:

    R=M(2U/O)I

Where:

M is the magnification constant of camera and optics;

U is the distance from object to cornea;

I is the observed size of image; and

O is the actual size of object.

The objects imaged are the several rings of the target which yield thecurvature of the eye at several distances from the center of the cornea.For the ith ring, all the constants are lumped into one, K_(i), thus:

    R.sub.i =K.sub.i I.sub.i

R_(i) is the radius of curvature of cornea of the i^(th) ring;

I_(i) is the observed size of i^(th) ring; and

K_(i) is the ith ring conversion constants.

Thus, all that is needed for computation of curvatures are the K_(i)constants. The K_(i) 's can be calculated but it is much easier, andmore accurate, to measure them by calibrating the instrument with ballsof known, precise diameter R_(o) and setting all K_(i) =1. The values ofV_(i) are measured which provides a measurement of I_(i) since V_(i)=1×I_(i). Thus, the constants are determined by:

    K.sub.i =R.sub.o V.sub.i

Where:

R_(o) is the known radius of calibration ball; and

V_(i) is the measured radius of calibration ball with K_(i) set to 1.

According to conventional techniques a table is constructed to provide alook up system for conversion of measured reflex diameters, representinga range of known surface curvature values. In this manner the necessarydegree of precision may be achieved to assure accurate output dataaccuracy for the intended application. Interpolation between tableentries is quite practical and reduces the number of table entriesneeded to assure accurate measurements.

A more exact surface shape characterization could, in theory, beobtained by the method iterated by Wittenberg and Ludlam in a paperpublished in the Journal of the Optical Society of America Vol. 56 No.11, November 1966 but the simpler form provides adequate accuracy forclinical use. The magnification factor and the effective numericalaperture are chosen as a compromise between the most desirable smallrelative aperture and acceptably small depth of field to facilitate thesetup and focusing step. This provides an acceptable error from subjectpositioning resulting from inability to judge small differences insubject distance due to the depth of focus of the optical system as wellas adequate image brightness for noise reduction. In most, if not allcases, the exact surface contour is of less interest to the clinicianthan the relative contour. For example, in a surgical application, theobject is to arrive at a smooth, regular corneal surface, which has asimilar shape in two perpendicular axes. That is to say that the cornealastigmatism is minimal. The errors of measurement are least at, or near,the center of the cornea which is the main image forming surface of theeye. Therefore small error accumulation in the periphery of the corneaare tolerable. In surgical procedures where the cornea is cut, suturetension and location can alter the surface shape. The peripheralcurvature must be maintained as closely as possible to the same value inall axes if there is to be no induced post-operative astigmatism. Thekeratometer can provide information for post-operative adjustment ofsutures to better achieve this result. The shape derivation for contactlens fitting is also a comparative process in that the lenses may alsobe measured by the instrument and so small errors from true surfacederivation cancel and the resulting data are usable in a clinicalcontext.

Because the eye is centered in the picture by adjustment of theinstrument and headrest at the time of setup and because the subject isfixating on a target which is coaxial with the system's optical axis,the center of the reflected image and thus, the cornea can be locatedexactly by a rather simple software technique. The largest differencebetween the two central image points from the fixation lamp reflectionconstitutes a measurement that is equivalent to a diameter of the innerPlacido ring reflection (in pixel terms). One-half of that measuredvalue is the center of the figure. The remainder of the analysis isbased upon similar technique and is much less software intensive thanthe classical image analysis algorithms which make more complexdecisions about a much larger number of pictorial elements each of whichmay have one of many numerical values which may represent intensity,saturation and hue. Thus it can be seen that this system substitutesnovel means and method for the conventional image analysis technique topermit the construction of a very inexpensive system which can be usedto produce clinically useful data when operated by unsophisticated userswithin the economic constraints imposed by current clinical feestructures.

The computer program controls measurements, data analysis and displayformat. Each single measurement consists of measuring the edges of thePlacido reflection in view. Subsequent to the data gathering step, thecurvatures are computed from the available edges. Any values fallingoutside of a window of selectable size are considered "bad". Then thehalf chord measurements for each ring from the selected data points arederived. The values of curvature are similarly computed for each ringimage on each side of center at enough angles to permit accurateassessment of major and minor axis angles.

The formula used for computing the curvatures is:

    R.sub.i =K.sub.i R.sub.i

Where:

R_(i) is the radius of corneal surface curvature of i^(th) ring;

K_(i) is the lumped constant of i^(th) ring; and

R_(i) is the measured radius of i^(th) ring.

(The lumped constant depends on magnification, ring size, local rate ofcurvature etc.) The constants K_(i) are determined by calibrating theinstrument by measuring objects of known radius. These data are storedon a disk, in an EPROM (Erasable Programmable Read Only Memory). Forsome similar means for use by the main program. The provision of avariable focal length camera lens would permit adjustment to compensatethe magnification errors which will result from the tolerance of focallength of commercial lenses if desired but the calibration table methodis the preferred embodiment.

II. Processing Circuitry And Operation

Referring now to FIG. 2, the keratometer of the preferred embodimentcomprises a Placido or similar target, a lens system, a CCD (ChargeCoupled Device) camera 50 for receiving the reflection of the Placido 2from the eye and an image processing sub-system 48.

The eye to be examined is positioned according to conventionaltechniques preferably at a distance of 3 inches from the Placido andcentered on the optical system. Referring to FIG. 15 for more detail thePlacido is in the form of a trans-illuminated surface of translucentmaterial 220 with the CCD camera lens centered in the Placido, and withthe lens, in turn, surrounded by concentric circles of opaque material221. The Placido is illuminated by one or more lamps 222 placed behindthe disc surface so the translucent areas are bright circles as viewedby the subject. By this technique an image is provided in a plane 223posterior to the normal corneal surface of the eye 224. Referring backto FIG. 2, the reflection of this image is received by a CCD cameraafter passing through the lens. The lens preferably includes anobjective lens 53 located at or near its focal length from the eye. Abeam splitter or mirror 15 may be included along with a second lens 52whereby a portion of the image formed by the objective lens may bediverted to a camera port for photographic recordation of the eye andthe Placido reflection. Otherwise the remaining image portion is broughtinto focus at the photo-sensitive surface of the CCD camera 51.

The subject is placed in front of the instrument with the chin supportedin a rest (24) which may be adjusted for subject size in terms of chinto eye dimension. This adjustment (25) is typically a screw operateddevice. The optical assembly (58) is mounted in suitable "slides" orrollers (45, 46) which permit motion in two perpendicular planes withoutrotation so that either eye may be aligned on the optical axis and theimage brought into critical focus by the motions. The Placido (2) isilluminated from behind by a lamp (27) which may be a circularfluorescent tube or other type as desired. The assembly is also moveablein the vertical axis which could be by a means of a slide (32, 33) undercontrol of a screw (31). The rotation of the screw may be by a knob or amotor drive comprising a motor (40), pulleys (37, 39) and a cooperatingbelt (38) or other suitable means to permit the elevation of the opticalaxis to be under operator control for alignment of the optical axis ofthe instrument with the eye to be measured. The action of a "joy-stick"(42) mounted in a ball and socket system (43) under operator control viaa cam or friction member (44) preferably propels the instrument on theslides, rollers or wheels (45, 46) to facilitate the positioning andfocus steps. The present system in accordance with the inventionutilizes a positioner assembly from SCO, Scandicci, Florence, Italy. Abrow rest (26) may be mounted on the head support system (49) to insurethe fixed position of the eye to the instrument while the adjustment andmeasurement arc made. The patient is requested to focus his eye on thefixation target (79) to assure the coincidence of the optical axes ofthe instrument with the eye (1). After the positioning and focus step,the operator presses the switch (41) or a foot operated switch, at whichtime the portions of the image relating to the measurement to be madeare captured by the electronic assembly (48) and suitable power supply(47) operatively associated therewith.

An object (the Placido) is reflected from the surface of the cornea andthe size of the reflection is measured. The focal length of a convexmirror is one-half of the radius of curvature and the image and objectsizes can be related to the focal length. The object in this case ispreferably a Placido or Placido's disc.

The data points which are recorded in memory comprise pixel numberswhich denote the locus in X, Y terms of each brightness transition inthe picture which are over threshold magnitude. These points arecontaminated to some extent by random noise and so must be treated toremove the noise, establish centering and focus accuracy and generalquality prior to being converted into final form for use in standarddisplay algorithms. As the data points are stored in memory at the timeof recordation, the points which define line numbers can be identifiedby addition of a flag bit in the position commonly occupied by the signbit. This is possible because the data points all bear a common positivesign and makes sorting simpler by making use of a sign compareinstruction available in most computers. The line numbers are stored asa paired table with the data points provided by the pixel numbers in themeasurement and the process continues until all data points are sosorted. The end of data in storage is indicated by either a line numberor a pixel number being equal to zero which is caused by clearing theentire data memory to zeros prior to each measurement. This techniquereduces the number of data points to be treated in the ensuingcalculations. A numerical mask is set into the software to define asmall area at the center of the picture which defined the location inwhich the fixation target reflection will be found if the instrument isproperly aligned with the eye (1). The reflection of the fixation targetshould be inside this mask for best accuracy. The data points within aslightly larger area are averaged to define the optical center of thedata to be treated.

If the average data point is within the mask area, it is stored as thecenter point for polar data form conversion; if outside the mask themeasurement is aborted. The operator may be notified of the error or anautomatic repeat measurement for some given number of tries, commonly(3) can be done prior to the notification as desired.

After the data format conversion from Cartesian to polar form the anglecount is set to zero and the points in radial sequence are stored in atable. This is repeated for as many angles as are desired. The increasein the number of angles enhances later display use but increases thecalculation time so the number of angles is user selected.

After all desired angles have been converted, the data points areexamined by distance from center as groups. It should be noted that thisis in sequence terms as opposed to discrete distance terms in that thereflection will be closed, nested curves, but not circles or otherregular figures in most cases.

The radially selected groups are subjected to a smoothing process suchas least squares or moving average window to define the shape of thereflection of the Placido. To provide the common form of central K1, K2,Cylinder and Axis, the innermost smooth curve may be presumed to be anellipse and the calculations produce the "best fit" ellipse from thesmoothed data. From this the K1 and K2 are determined by look up andinterpolation from the calibration data table and the numericaldifference becomes "Cylinder" or astigmatism. The Axis is, of course,the major axis angle of the determined ellipse in anti-clockwise formfrom zero degrees in the horizontal plane extending to the right of theorigin.

The remaining steps take each set of points for successive concentricreflections and smooth them in like fashion. Any data point which failsto fit the smooth curve by more than two standard deviations or otherlike threshold parameter is then deleted and the data resmoothed. Thesmoothed data are then converted to X, Y and millimeter radius ofsurface curvature form by table look up for use in any desired displayformat.

The area of corneal coverage is 0.9 mm-9.0 mm. (@42.5D). The axis rangeis 0-360 degrees (1 degree increments). The diopter range is 9D-99D. Theresolution is ±0.25 diopters. The dimensions of an integrated systemembodiment are 23" D×18" W×24" H, 80 pounds, otherwise, the systemcomponents can be modularized and provided on a compact mobile pedestaltable.

SYSTEM COMPONENTS

The system in accordance with the invention is comprised of aphotokeratoscope, a Placido 2, a patient focusing assembly 202, acomputer 203, a high resolution CCD video camera, a 14" VGA colormonitor 200 and an image processing subsystem. The system is mounted ona table top which is attached to a moveable pedestal 205.

The illustrative system in accordance with the invention includes thefollowing components:

Photokeratoscope, case, CCD camera, Placido, light chamber, opticsassembly, patient focusing assembly, positioning base/chinrest, IBM ATcompatible computer or 80386 based computer, 101 Key Enhanced Keyboard,40 Megabyte Hard Disc Drive, 1.44 Megabyte Floppy disc Drive, HighResolution CCD Video Camera, 14" VGA Color Monitor, Image ProcessingSub-System, Image processing algorithms, frame grabber board, powersupply board, pedestal, tabletop.

IMAGE PROCESSING

The image processing software and all other software used in the systemin accordance with the invention is set out in the appendix. Thesoftware is adapted for speed and performance in numerous ways. Thesoftware uses integer mathematics in lieu of floating point mathematicsto obtain a substantial increase in speed on the family of processorsused by the system, the Intel X86 family of processors, available fromIntel Corporation, Santa Clara, Calif. These techniques improveperformance on any processor, however. Integer math is even faster thanusing a co-processor for floating point operations. The math uses afixed point operator. For instance, to use the number 3.279 the proxynumber 3,279 is manipulated instead using integer math; the decimalpoint is later placed in the result as necessary. This is much fasterthan floating point math. The system in accordance with the inventionalso uses integer math for sines and cosines, simply scaled by 1000 togive a significant increase in performance. Because only threesignificant digits are necessary, this scale by 1000 operation worksadequately to give three significant digits.

Numerous performance enhancements are detailed in the source listing. Animportant factor in performance enhancement is the architectural designof the software as well as the selection of steps and sequence used toperform the image processing and other function. Additionally thetechnologies of image processing, parallel processing and expert systemsare incorporated into the software design.

The software design is parallel. It can be executed on a parallelprocessor such as a super computer and would not be forced to besequential. Therefore the architecture has been designed so that it canbe executed in a parallel implementation.

EDGE DETECTION

The system in accordance with the invention uses an edge detectionalgorithm implemented in software. Each Placido ring that is reflectedin the cornea is seen as two edges by the edge detector. Other knownsystems use peak amplitude to detect Placido ring locations which isless accurate and generates fewer data points for post image captureanalysis. The system in accordance with the invention uses the edgedetection software to sense the interior and exterior edge of each ring(see FIG. 12K). The image processing software then counts the number ofpixels to each edge of a Placido ring, and then rotates 1 degree andrepeats the process of counting pixels.

For example, if nine Placido rings were used and reflected off of thecornea they would generate eighteen ring edges; this implies 360degrees×18 edges=5760 points of corneal topographic information. Olderstyle keratometers only utilized four data points, the radius to asingle mire or ring measured 90 degrees apart and then photographed themire reflected in the cornea to quantify a spherical characteristics ofthe cornea. The total analysis alone took over 20 minutes to complete.The system in accordance with the invention performs a full 360 degreeanalysis in under 15 seconds.

The number of pixels counted to the edge of a Placido ring correspondsto a particular radius of curvature when compared to the calibrationcurve for the system. The edge detector which resides as software in thecomputer looks at the pattern of rings reflected from the cornea on theCCD camera and captured by the frame grabber board and counts the numberof pixels to the edge of each concentric ring. The number of pixelscounted to each ring edge is proportional to the radius of curvature ofthe corneal surface of the eye at that point.

The image processing software resides in the integrated computerperforming edge detection to find the edge of each Placido ringreflected on the corneal surface, then image processing algorithmsoftware creates a table of pixel distances to each Placido ring edgethus generating a pixel count or distance to a ring edge that isproportional to the corneal radius of curvature. The system inaccordance with the invention uses sub-pixels 1/10 resolution todetermine position of edges and diopter measurements. These calculatedpixel distances are compared to the calibration curve to generate thetopographic curvature for the object cornea.

Software also includes function for patient history, data basemanagement, displays, driving the video board, writing pixels to thedisplay board buffer, site specific profiles for communicationsparameters, doctor preferences for number of colors on the screen, filemanipulation code, menus and numerous other functions evident uponexamination of the software source listings in the appendix.

CALIBRATION CURVE

The illustrative system in accordance with the invention generates acalibration curve by imaging objects with a known radius of curvature.The calibration routine calculates and stores a look up table(essentially a calibration curve) for each of four calibration spheresin the current design, each table corresponding to the number of pixelscounted for this known radius of curvature. The number of calibrationspheres can easily be increased or decreased. Presently these fourtables are used to generate an interpolated calibration curve (pixelversus diopter or radius of curvature). This best-fit curve, presentlycalibrated to four known radius-of-curvature calibration objects, givesthe radius of curvature for a given pixel count when imaging an objectwith an unknown radius of curvature. The software source code is listedfully in the appendix.

DATA PRESENTATION AND DISPLAY

Corneal information can be reported as a set of numerical values or maybe displayed in a color-graphic presentation. The system in accordancewith the invention is capable of graphical presentation of the 3 mm, 5mm and 7 mm zones, or as a corneal contour profile graphic of anymeridian. The system in accordance with the invention can also generatetopographic color coded surface maps in either diopter or millimeterradius of curvature scales. Up to four surface maps can be displayedtogether for comparative analysis. Patient exams can be archived tofloppy disc and recalled at any time. Permanent records can be producedvia optional Polaroid camera or color graphics printer.

The colored graphics presentation can be utilized to show where to makecorrecting incisions into the cornea during a radial keratotomy or lasersculpturing procedure of the cornea. A video display monitor is utilizedto view the graphic presentation. Video graphics can be saved on thesystem printer or on disc for archival purposes.

Graphical displays can also be useful to record the topographic historyof the cornea during the healing process. The cornea can take months andsometimes even years to heal: the cornea has no blood in it, so itrepairs similar to a missing finger nail; it does not scab over and healwithin a week or so. Such historical topographic data enables a doctorto make necessary adjustments as the cornea heals. The doctor cantighten or loosen sutures or make other correcting adjustments tooptimize the corrective effects of surgery on the shape of the cornea.

Graphical presentations can also be used to compare the topographiccorneal characteristics before and after surgery. The difference betweenthe two can also be shown so that a physician can observe how thesurgery has affected the topography of the cornea. Examples of thegraphic presentations are presented in FIGS. 3A-3L.

The graphic display can be chosen from menus as shown in FIG. 3A usingthe "Select Display Format" menu. FIG. 3B is an example display ofkeratometric data (orthogonal). FIG. 3C is an example display ofkeratometric data showing astigmatism in a "torque display" in which the3 mm, 5 mm and 7 mm topographies are overlaid in one display. FIG. 3D isa profile graph which can be generated for any 2 meridians. FIG. 3E is atabular display of keratometric data which can be generated for any 2meridians. FIG. 3F is an example display of a contact lens fitting mapshowing the dioptric correction for different points on the cornea. FIG.3G is an example of a comparative isodioptric mapping that can begenerated to compare 1, 2, 3 or 4 eyes. FIG. 3H is a color map withnormalized dioptric scale. FIG. 3I is an example of a data overviewdisplay. FIG. 3J is an example of display eye image which is a displayof the eye and the Placido ring image upon it. FIG. 3K is an example ofthe contact lens fitting display. FIG. 3L is an example of comparativeiso-dioptric color mapping. FIG. 3M is an example of a tabular displayof curvature data in any two selected meridians.

The data organization and presentation software source code is listed inappendix 1.

PRECISE PLACIDO POSITIONING AND FOCUSING AID

The size of the reflection of the Placido in the cornea, and thereforethe calculated perceived radius of curvature for a particular zone ofthe cornea, is a function of the distance from the eye to the Placido.Therefore it is desirable that this distance be the same each time thedoctor analyzes the topography of the cornea. A change in the distancewould render an erroneous calculation as to the topography of thecornea. For instance, should the Placido be positioned slightly closerto the cornea on a second "snapshot," the rings would appear fartherapart and more pixels would be counted between the rings even though theshape of the cornea has not changed, and the results would erroneouslyindicate an increase in the millimeter radius of curvature.

In the present embodiment the actual Placido is approximately 3" fromthe eye; however, a focus aid is employed to exactly position thePlacido to the same position for each diagnostic session. The focusingaid is much closer to the eye than the actual Placido and it projectsfocusing cross-hairs on to the eye. These focusing cross hairsconstitute a "synthetic Placido" which represents a Placido which ismuch closer to the eye than the actual Placido. Therefore when thefocusing aid or "synthetic Placido" is positioned precisely thepositional errors in the actual Placido are negligible. Therefore thedeviations in the position of the actual Placido become insignificantand aid accurate and reproducible positioning of the actual Placido,thus reducing inaccuracies in corneal topographic calculations due topositioning errors.

The focusing aid projects focusing cross hairs onto the eye. Thefocusing aid acts as an optical range finding system to determine whenthe cornea is in focus. The focusing aid promotes accurate andreproducible eye placement to get exact comparative readings betweenpre-operative and post-operative corneal topography. Comparativereadings are also useful in determining how the corneal shape may changeover time during the healing process. The position of the Placido withrespect to the eye is important in determining the corneal topography,in both an absolute or a comparative sense. The corneal topographiccharacteristics are also useful in determining and predicting the aftereffects of surgery and detecting possible errors that may have occurredin surgery using other diagnostic techniques than the system inaccordance with the invention.

The space between the reflected Placido rings is a function of thedistance from the eye to the Placido 2. Because the Placido may appearin focus during travel through the depth of field for a particular lens,there can be significant variance in the distance from the Placido tothe eye for two different points within the depth of field. A differencein this distance from the Placido to the eye causes a difference in thedistance between the Placido concentric circles, inducing an error inmeasurements of the distance between the Placido lines. The Placidoshould be positioned at the same distance from the eye each time ameasurement is taken so that variations in the distance betweenreflected Placido lines are caused by variations in corneal topographyand not by variations in the distance from the Placido to the cornea.

Now referring to FIGS. 4A and 4B, prior art systems use a triangulationmethod (100) as shown in FIG. 4A. The prior art method employsconverging laser beams 100 from lasers 102, to position the apex of thecornea, 101 relative to the optical assembly 58. This method introducesan error in post-surgery keratometric readings, as the tip or apex ofthe cornea 101 may be depressed substantially from its pre-operationposition. This depression causes a flattening of the corneal apex asshown in FIG. 4B. This flattening causes the cornea to be positionedcloser to the optical system under the prior art and thus exaggeratingthe corneal flattening resulting from the surgery. The prior arttriangulation focusing method induces an error in the distance to thereference point, the apex of the cornea. The prior art system thereforeis less likely to give consistent and repeatable results or measurementsbecause the distance from the optical assembly to the entire corneachanges after surgery inducing an error in postoperative measurement.Moreover, the use of lasers projected onto the cornea is also dangerousas they can damage the tissue.

The illustrative system in accordance with the invention offers animprovement in that these induced errors are reduced to enhancerepeatable and accurate results. The present system in accordance withthe invention uses two light emitting diodes (LEDs) 104, however anotherilluminating source or means of projecting an image could be used. TheseLED projected images do not converge but are pointed at the limbus areaat the periphery of the eye. These LEDs project a focus aid imageconsisting of an "x" or cross hairs 103 onto the outer portion or limbusarea of the eye. This outer portion is less susceptible to changethrough either flattening or steepening than the apex area of thecornea. The change in this limbral area of the cornea after surgery isnegligible compared to the change in the apex. Therefore themeasurements are more accurate and comparable using the system inaccordance with the invention of the present system for pre and postoperative corneal curvature changes.

FIGS. 5A, 5B and 5C show the focusing aid in detail. An LED 121 is heldin place by an LED holder 122. The tube 126 encases the focusing aid. Aspacer 123 fits inside the tube 126 along with lens one (f=84 mm) 125and lens two (f=48 mm) 124. The focusing aid is secured to the opticalassembly 58 by the focusing aid mounting collar 127. The LED holder 122has 38 gauge wire cross hairs 129 attached with epoxy. Focusing aid 130projects these cross hairs 129 onto the limbus area of the eye. Thecross hairs 129 are reflected by the eye back into the optics system anddisplayed on a video monitor for the operator to observe. In FIG. 9, theoperator actuates joy stick 42 bringing the Placido 2 into focus. Theoperator via the joystick moves the optical assembly 58 along theoptical axis 151 as shown in FIG. 6A. This motion moves the opticalassembly 58, focusing aid 130 and Placido 2 along the optical axis. Theoperator observes the focus aid image 103 in FIG. 4A on the videomonitor 200 in FIG. 1. The focus aid image is reflected off of thelimbus region of the cornea into the optics assembly into the camerawhere it is displayed on the video monitor 200. The angle of incidence170 in FIG. 6A of the focus aid image upon the eye changes as thefocusing aid travels along the axis 151. When the angle of incidence isproper, the cross hairs split the circle projected from the focusing aidinto equal quadrants and the focusing aid is properly focused at thecorrect distance to take a shot of the Placido properly focused on thecornea. That is when the focus aid is at the proper distance and infocus as indicated by the cross hairs positioned shown as in FIG. 6C,the Placido is also at the correct distance and properly in focus. Thistechnique gives repeatable and consistent results.

When the cross hairs 129 in FIG. 5B are seen as shown in FIG. 6C thePlacido is focused at a repeatable distance from the eye each timebefore and after corneal surgery because of this precise focusingtechnique upon the limbral region of the cornea. Changes in the centralshape of the cornea have a negligible effect upon the focus distance andtherefore introduce little error into the analysis of the cornea. Hencethe measurements are repeatable and negligible error is induced by achange in the reference point. The operator adjusts the focusing aid andplacid reference distance using a calibrated sphere with a known radiusof curvature.

FRAME GRABBER BOARD

The present system in accordance with the invention takes a digitalpicture of the Placido as it is reflected by the cornea using the CCDcamera. The frame grabber board stores this image by grabbing two (evenand odd) consecutive NTSC video fields 1/60 of a second apart, storingthem in memory to form a NTSC video frame giving a composite image forviewing by the operator. The board also enables the operator to actuatethe frame grabber via a foot switch. The board is designed to work athigh speed with the computer and software. The design details andschematic, as well as the programmable logic array equations, arepresented in FIGS. 10A through 10K.

EDGE DETECTION AND ANALYSIS

The illustrative system in accordance with the invention looks at thePlacido reflection from the cornea and determines the position of theedges of the light and dark pattern generated by the Placido. The edgedetection and analysis software is set out in the appendix.

PLACIDO SELECTION AND DESIGN

Different types and shapes for the Placido may be used. In a cylindricalPlacido the rings are marked around the inside of the tubular surface togenerate a pattern of rings when projected onto the cornea. However, inthis arrangement the distance from the Placido to the eye is very short,usually less than 1" and most likely right on top of the eye. A planarPlacido increases the distance from the Placido to the eye and in thepresent embodiment is at approximately 3". Increasing this workingdistance from the Placido decreases the effect of positioning errors.That is an error of 1/10" is a much smaller percentage of 3" than it isof 1" so that as an error of 1/10" has much less effect on themeasurements using a planar Placido with a working distance ofapproximately 3", than a cylindrical Placido with a working distance of1".

The generic design of the Placido is set out in the mathematical modelbelow. In a planar Placido the inner bands of the Placido are thinnerthan the outer bands to generate a 50% duty cycle between light and darkedges in the reflected Placido image off of a normal cornea. The Placidocan be designed to generate a 50% duty cycle of light and dark edges inthe reflected Placido image or any other duty cycle or variable dutycycle desired. The Placido can be designed for any shape also using themathematical model set forth below. The design of the Placido is ageneralized design and is set out below as a mathematical model. Thismodel works for any shape Placido. The design model tells the operatorwhere to put the Placido edges on any shape Placido. For any shapePlacido the operator must mark edges on the shape and the mathematicalmodel tells the operator where to mark the edges on the shape.

The Placido generates a virtual image on the convex cornea. The imageactually exists behind the surface of the cornea, so when the operatorfocuses, the focus is on a point internal to the eye where the virtualimage exists. This virtual image of the Placido is object of the camera.The virtual image is a series of rings. Using this design method and aplanar Placido increases the working distance, which is more comfortablefor the patient and less difficult to position.

MATHEMATICAL MODEL FOR PLACIDO

Each larger successive concentric Placido ring is wider to reflect anominally uniform width set of rings in the cornea. The normal anglewith respect to optical axis at point yi (2.y1 dia zone) is given by a=sin -1 y1/7.937!, where 7.937 is the radius of curvature at a 42.5diopter surface. For a reflected ray from point y1 to be parallel to theoptical axis, it follows that the angle of incidence of that ray (onpoint y1) be

    <i a=<r.

This ray emanates from a ring edge on the Placido making the enter ofcurvature of a 42.5 diopter surface our origin in a Cartesian frame ofreference, we have the locus for the Placido point yielding a reflectionat the 2Y diameter zone as

(y-y1)=M (x-x1) (equation of a line)

M=Tan 2a (angle of incident ray with respect to x axis)

Since (y1) 2+(x1) 2=(7.937) 2 (equation of 42.5 Diopter surface)

Therefore, Y=X tan 2a+ Y1-(tan 2a) (7.937 2-Y1 2) 1/2! ##EQU1## is thefocus of Placido points yielding a 2 Y1 diameter reflect on a 42.5 Dsurface.

The tip of the 42.5 D surface is at 7.937 mm=0.3125".

Choosing an x (eye clearance is x-0.3125 inches) yields an ordered pair(X,Y) for the Placido profile. Note that Y Max will be the overalldiameter of the Placido (X Max, Y Max) if Y1=Max desired zone covered.Choose Y2=Min desired zone covered Y=7/8"/2 yields (X Min, Y Max) forPlacido inside circle for a conical Placido, the focus of Placido pointsis ##EQU2## and the solution of 4)+3) yield edge radii. NOTE

Conical Placido profile has been used here but the theory obviouslyextends to any desired profile, from cylindrical to planar

OPTICAL ASSEMBLY

The optical assembly houses a power supply and electronics, illuminatinglamp, a camera, the optics, a Placido and the focusing aid. The camerasits inside the optical assembly and behind a plate. The camera has anoptical tube that contains a lens which passes through the plate andprojects all the way forward to the Placido. The tube surrounds theoptical path of the Placido image. The fluorescent lamp that illuminatesthe Placido sits in front of the plate. The shape of the housingeliminates the need for a reflector pan as the housing serves as areflector behind the lamp that illuminates the Placido. The purpose isto have a homogeneous light source to illuminate the Placido.

The optical assembly is detailed in FIGS. 8A-8I. The layout and designof the optical path is set out in FIGS. 11A and 11B. The optical path inthe current embodiment is a single lens system taking a magnification of0.58 for 12 mm coverage so the doctor can see slightly more area thanthe eye itself. The design can accommodate various lens sizes. Forexample for a 75 mm lens, the total path length of the tube isapproximately 8 inches. The outside diameter of the tube is 11/4" andhas 3 baffles with 3/4" apertures.

Magnification is important to the resolution of the system. The pixelresolution of the system is proportional to the magnification of thelens. More magnification means more pixels per millimeter and lessresolution. The more pixels per millimeter that are present the betterone can analyze small changes across that distance. For example, if youhave 5 pixels per millimeter you can resolve a pixel 1/5 of a millimeterin size. If you have 10 pixels per millimeter you can resolve a pixel1/10 of a millimeter in size.

POWER SUPPLY BOARD

The power supply board is specifically designed to work with the presentsystem in accordance with the invention and is presented in detail inFIG. 9.

CONTACT LENS FITTING SYSTEM

The system in accordance with the invention includes a contact lensfitting system including software in which the corneal analysisgenerated inputs into a transformation function operating in software.The transformation function converts the corneal topographic profileparameters into contact lens design parameters. These contact lensdesign parameters are sent to a contact lens lathe, well known in theart, to sculpt a custom contact lens to fit the eye that has beenanalyzed. The contact lens design parameters may be checked for qualitycontrol before sending the parameters to the lathe or the parameters canbe sent to the lathe without such a quality control checking functionand simply let the patient and physician determine if the lens issatisfactory. A software function (or equivalent) to transfer thecontact lens parameters from the corneal analysis computer to a lathefor sculpting a lens is necessary to implement the system. One suchquality control function has been developed by Polytech, Division ofEMI-MEC, Limited, a Sunleaigh Company, School Lane, Chandler Ford, EastLeigh, Hempshire, England, S05 3ZE. The present system in accordancewith the invention does not claim the Polytech version of the qualitycontrol function. The software for the transformation from cornealparameters to contact lens design parameters and the link software fromthe design parameters are available in source listing form in theappendix to the specification.

The communications software that sends a file from the cornealtopography analyzer computer to any other computer utilizes an off theshelf package available from Blaise Computing, Inc., 2560 Ninth St.,Suite 316, Berkley, Calif., (415) 540-5441.

CHECKERED PLACIDO

A ray passing through a point on a checkered Placido, reflected off acornea, and detected by a CCD camera is graphically depicted in FIG. 15.The focal plane 40 of the CCD camera, the plane of the lens 42, theplane of the Placido 44 and the plane 46 tangential to the apex of thecornea are depicted in FIG. 15. Each plane contains a local XYcoordinate system. The origin of the XY coordinate system existing ineach plane is intersected by a line 48 representing the optical axis ofthe eye. Each plane is parallel to the other planes. The optical axis iscoincident with the origin of the coordinate system in each plane. Point"A" 50 lies on the Placido 44. A line intersecting point "A" 50 and theorigin of the XY coordinate system lying in the Placido plane 44 formsan angle "a" 52 with the horizontal axis or X axis of the XY Placidocoordinate system.

As shown in FIG. 18 the Placido in an illustrative embodiment is shapedlike a cone. In an alternative embodiment, the Placido could be aparaboloid. In yet another alternative embodiment, the Placido could beyet another shaped surface. The patient looks into the concave surfaceof the conical Placido in a preferred embodiment. The exterior or convexsurface of the conical Placido in a preferred embodiment is backlit by alight source. Point A 50 represents a point on the Placido. A ray oflight from the light source will pass through point "A" on the Placidoand strike a reflection point 58. This ray is called the incident ray56. The incident ray 56 passes through Placido point "A'" 50 and isreflected at the reflection point 58. The reflected ray 60 is detectedpoint "A" 62 on the CCD focal plane 40. A line 64 passing throughdetected at point 62 and through the origin of the coordinate systemexisting in CCD plane 40 forms an angle "a¹ " 66 with the horizontalaxis or X axis of the CCD plane coordinate system.

Referring now to FIG. 19 a front view of the checkered Placido, thePlacido is laid out in a checkered pattern. In a preferred embodiment,the checkered Placido is made up of black and white sections. In analternative embodiment, the checkered Placido could be made up ofanother set of contrasting color. The checkered pattern is designed sothat black and white transitions are encountered when traveling along aradius drawn from the origin 74 to the outer edge 76 of the Placido asconcentric rings of contrasting color are encountered. The design alsoprovides for black and white transitions when traveling along an arc 78.The arc 78 is generated by angularly rotating a point drawn a distance Rfrom the origin 74 where R is less than the radius of the Placidoperimeter 76. Thus, edge transitions are encountered when travelingalong a concentric circle drawn inside the perimeter of the Placidocircumference as adjacent sections of contrasting color are encountered.These sections are formed by drawing a plurality of radii to form thetriangular shaped sections shown in FIG. 19. Upon drawing a plurality ofradii there will be edge transitions encountered both radially andconcentrically in the pattern. There will exist points on this patternwhich will have edge transitions both radially and concentrically. Thesepoints will be defined as nodal points where it is possible for directmeasure of orthogonal radius of curvatures, radially and concentrically.

A benefit of the checkered Placido is that, as shown in FIG. 15 with thecheckered Placido, the meridian of the incident ray can be determined byconstruction. Therefore when the meridian of the reflected ray ismeasured it is possible to determine the precise orientation of thesurface normal at the reflecting point. Referring again to FIG. 15, inthe past it was assumed that the incident ray, the surface normal andthe reflected ray were contained in a single plane. This plane wasassumed to contain the principle or optical axis. However, this is notnecessarily true. It depends on the precise orientation of the surfacenormal at the reflecting point or the shape of the surface at thereflection point.

When the reflection point is located on a perfectly spherical surfacethe principle axis or optical axis is located in the plane containingthe incident ray, the surface normal and the reflected ray. However,when the reflection point is located on a non spherical surface, such asa cornea having non spherical characteristics, then the optical axis isnot located in the plane containing the incident ray the surface normaland the reflected ray.

In the checkered Placido the angle or meridian of the incident ray canbe determined because the checkered Placido has a marking or identifyingline at the X axis in the coordinate system for the Placido plane. Thus,the deflection angle 79 of a line drawn through a point "B" 50 on thePlacido plane can be determined. Thus, the XY coordinates of the point"A" 50 on the Placido plane are known. At nodal points there will be aunique surface normal in three dimensions readily defined since the edgetransitions occur in orthogonal curvatures.

Referring now to FIG. 15, in a system with a Placido consisting only ofconcentric rings, not having the checkered pattern of the presentembodiment, it was assumed that the angle "a" 52 measuring the angulardeflection from the horizontal of the point "A" on the Placido plane,and the angle "a'" 66 measuring the angular deflection from horizontalof the point "a'" 62 the detected point on the CCD image plane, were thesame. That is, it was assumed that the angular deflection of the pointon the Placido plane and the angular deflection of the point on thedetected CCD image were the same. However, this assumption is notnecessarily true when the reflection point is located on a surface thatis not perfectly spherical.

The point "A" 50 is reflected at the reflection point 58 and passesthrough the lens center represented by the origin of the lens planecoordinate system 42 and forms an image on the CCD focal plane. There isalso a parallel ray from a virtual image, which is behind the eye. Theparallel and the principal ray or chief ray converge at point 62 todetermine where the image is formed. The system graphically depicted inFIG. 15 has been designed to form the image at the CCD focal plane sothat only the principal ray is a concern in the calculation.

Previously it was assumed that the angle 52 was equal to the angle 66because there was no way easy to determine where the point 50 waslocated in the Placido plane 44. However, with the checkered Placido thelocation of the point 50 on the Placido plane can be determined becausethe meridian or the angle "a" 52 is known by construction. Thedeflection angle or meridian 66 of the detected point can be measured onthe detected CCD image. The angle "a" 52 is known by constructionbecause it lies on or near the intersection of a black to white or colortransition edge on the checkered Placido. The angle measured fromhorizontal to each edge of the black to white or color transition on thePlacido is known. Thus, points on or near these transition "edges" canbe determined.

The deflection angle "a'" 66 of a detected point can be measured on theCCD. The angle 66 of the detected point will not equal the angle "a" 52of the point on the Placido when the reflection point is located on anonspherical surface. Also, when the reflection point is located on annonspherical surface, the surface normal will be twisted such that thesurface normal at the reflection point 58 will not be contained in thatplane containing the optical axis. It will be contained instead in askewed plane. When the angle 52 is not equal to the angle 66, thesurface normal is not contained in the plane of the optical axis and thesurface containing the reflection point is not spherical.

Therefore the location of the point on the Placido helps determine moreprecisely the shape of the eye. The checkered Placido helps to determinethe location of a point on the Placido by construction. The most obviouspoints are nodal points where orthogonal edge transitions occur byconstruction. The determination of the unique surface normal in threedimensions is a direct consequence of knowing the tangent plane at eachnodal point common to each of orthogonal local radius of curvaturemeasurements calculated from the respective edge transition locations.The location of any point on the Placido can be determined because thePlacido is constructed of black and white or contrasting sections whoseedge transitions can be detected and mapped or located so that a map isformed of these known locations of points on the Placido. The angulardeflection from horizontal for each black to white or color transitionedge is known because the checkered Placido is manufactured with a knowndeflection angle for each of these transition edges. Knowing themeasured angle 66 of the detected point, one can use solid geometry togo from point 62 on the CCD image through the lens center back to thereflection point on the cornea of the eye and back to a known point "A"on the checkered Placido. The angle 52 determines the surface normal ofthe reflection point in three-dimensional coordinates. Thesethree-dimensional coordinates define precisely the orientation of thesurface normal at the reflection point (N_(x), N_(y), and N_(z)). Thistriplet identifies the surface normal at the reflection point. Thesurface normal triplet can be determined for every point on the corneawhere a measurement is taken.

Any line originating from the center of a sphere and intersecting thesurface of the sphere is a normal. In nonspherical volume there may be asignificant perturbation away from a triplet for a normal on a sphericalsurface. The checkered Placido helps to determine this perturbation ordelta. Determining this perturbation or delta enables the cornealanalysis system to determine more precisely the topographical ornon-spherical characteristics of the surface of the cornea at thereflection point.

Points on the Placido arc mapped based on the angular deflection of thepoint from that zeroth meridian or horizontal axis. Each edge at a colortransitions is a known number of degrees from the horizontal. The radialPlacido sections are constructed by drawing radii so that each sectionis a known number of degrees from horizontal. For example, if eachradial section is 10 degrees wide the angular deflection between thehorizontal or zeroth meridian and the edge of the first section would be10 degrees, 20 degrees to the second section edge, 30 degrees to thethird section edge, and so on. The black to white edges or colortransition edges formed by the adjoining radial sections encountered inthe angular direction are detected by the same edge detection andlocation method as used for the edges of the concentric circlesencountered in the radial direction. In a preferred embodiment, adjacentsections on the Placido are alternately black and white, however, theadjacent sections can be another set of contrasting colors, as long as adetectable edge is formed between adjacent sections, to facilitatelocating points on the Placido.

The edges or color transitions are determined by the mathematicalprocess of differentiation. The derivative function highlights theseedges by generating an impulsive change. The impulsive change is used todetermine the precise position of the edge of the black to white orcolor transition between two pixels. The position is determined to asub-pixel position by a process of weighting and using the surroundingpixel information to determine where between the two pixels the edge isprecisely located.

The surface normal is calculated by drawing a line from the detectedpoint 62 to the reflection point and drawing a line from the reflectionpoint to point 50 on the Placido. The angle bisecting the angle betweenthese two lines is the surface normal.

A successive approximation process is used to determine the reflectionpoint at which the surface normal intersects the eye.

Knowing the location of the detected point 62, the lens center point andthe location of the Placido point 50, and reflection point one cancalculate the surface normal. A line is drawn from point 62 through apoint at the origin of the lens coordinate system 42 or the center ofthe lens. This line 60 is extended to intersect the plane of thereflection point. A plane which contains the line extended through point62 and the center of the lens is rotated until it touches the point 50on the Placido. The X and the Y coordinate of the surface normal aredetermined. The Z coordinate of the surface normal is then determined.Once the three coordinates of the surface normal are known-for a numberof points, a plane can be drawn orthogonal to each surface normal. Theplanes can be joined together to form a multi-faceted surface area. Thisarea of facets or joined planes can be smoothed to represent the surfacecontour of the cornea. That is, the faceted surface is integrated tosmooth the faceted surface to represent the actual contour of thecornea. Points in between the surface normal are calculated by theprocess of interpolation. The actual contour of the cornea can befurther described; for example, by calculating, among possiblecurvatures, the mean curvature, either arithmetic or geometric(Gaussian), at an infinitely small area when two orthonogal or principalradius of curvatures are known. The local area surrounding an nodalpoint can be directly analyzed to provide these mean curvaturemeasurements either arithmetic or geometric (Gaussian) to describe moreprecisely the local radius of curvature of the cornea's contour. FIG.demonstrates this calculation where a nodal point analysis containesorthogonal edge transition for determining the local radius of curvaturein orthonogal direction which can be mathematically manipulated toprovide the arithmetic or geometric (Gaussian) mean of the Icoal radiusof curvature to provide more precise analysis of the cornea's contour.This process provides novel capability to determine the contour of thecornea and therefore its refractive power as an optical elementnecessary for vision by the eye. Historically, curvature measurementshad only been made in a radial fashion from the optical axis of thecornea outward, peripherally. Mean curvature calculation frommeasurement of orthonogal local radius of curvatures will furtherenhance our understanding of the corneal contour and its shape inaddition to its optical performance. This process more precisely definesthe surface of the eye. ##EQU3##

At times it may be difficult to know exactly what ring on the Placidocorresponds to the ring detected in the CCD digital image. This may bedue to corneal distortion, surgical scarring, or some other opticalaberation that obliterates data in the digital image so that an edgebecomes undetectable. As explained above, it is advantageous to knowwhich ring or section on the Placido corresponds to the ring or sectiondetected in the CCD digital image. Ring seven may be mistaken for ringsix, for example, if the edge between ring five and six is not detectedand not counted, thus skipped. In an illustrative embodiment, as shownin FIG. 19, a reference mark 76 or numeral is placed on the Placido to aring or section on the Placido.

As shown in FIG. 20, reference marks or numerals enhance correlation ofpoints on the CCD digital image and corresponding points on the Placido.In an illustrative embodiment, reference marks or numerals are placed ineach white ring. Four lines of reference numerals are placedrespectively at zero degrees, ninety degrees, one hundred eightydegrees, and two hundred seventy degrees to facilitate checking of eachof the quadrants.

Even though the reference marks may undergo distortions, includingscale, perspective, and rotation, when reflected of a non-sphericalsurface, the system can find the section containing the reference mark.The system will do a mapping back into a normalized space to obtain anormalized detected reference mark. In normalized space there is alibrary in which templates are stored for the reference marks ornumerals as detected when reflected from calibrated spheres. The systemcorrelates the stored template mark or numeral from the library with thenormalized detected reference mark or numeral to confirm which ring orsection on the Placido corresponds to the detected ring or section.

The normalization of reference marks allows the system to recognizenumerals or marks that have undergone translation, rotation, andrubber-sheeting type of perspective distortion.

While an embodiment of the present system in accordance with theinvention has been described herein, it will be understood that a personskilled in the art may make minor alternations or substitute circuitryand apparatus other than that described without departing from thespirit of the invention. For example, those of ordinary skill having thebenefit of this disclosure will of course recognized that the"hard-wired" discrete logic function described herein may alternativelyand equivalently be implemented in software, i.e., through suitableprogramming of a processor system equipped with a suitable processor anda memory or other storage device. Such a software implementation wouldbe a matter of routine for those of ordinary skill having the benefit ofthis disclosure and knowledge of the processor system in question.Software functions disclosed in this application could likewise beimplemented in hardware by a person having ordinary skill and thebenefit of this disclosure.

What is claimed is:
 1. A method of corneal analysis using a checkered placido comprising the following steps:a) projecting the image of said checkered placido onto a patient's cornea; b) detecting an image of said checkered placido reflected off of said cornea; c) detecting a plurality of nodal points from said reflected image of said checkered placido; d) determining mean curvature of said cornea at a plurality of said nodal points; and e) analyzing said mean curvature at said plurality of said nodal points to produce a graphic display of the estimated actual curvature of said cornea. 